Leukocytes play crucial roles in the defense of the normal lung and in the pathophysiology of many common pulmonary diseases. Leukocyte adhesion receptors are of central importance in leukocyte trafficking and in the inflammatory and immune responses. The long-term objective of this project is to understand the adhesive interactions which are important in the recruitment and function of lung leukocytes. Many of these interactions involve members of the integrin family of cell adhesion receptors. The novel lymphocyte and eosinophil integrin beta subunit beta 7 appears to have a special role in mucosal immunity. The proposed experiments will further investigate ligand binding properties of the two known beta 7 integrins, alpha4beta7 and alphaMLAbeta7 (HML-1). alpha4beta7 is a receptor for fibronectin, VCAM01, and a previously unidentified ligand selectively expressed on mucosal venules. Preliminary studies indicate that this ligand is the mucosal addressing (MAdCAM-1), a molecule previously shown to be crucial for lymphocyte homing. Aim (i) is to characterize alpha4beta7-mediated binding of lymphocytes and eosinophils to MAdCAM-1. In vitro cell adhesion assays will be used to analyze the effects of function-blocking antibodies, cell activation, and other variables on alpha4beta7-dependent binding of leukocytes and beta7- transfectants to MAdCAM-1. Aim (ii) is to use heterologous expression systems and monoclonal antibodies to develop strategies for selectively modulating alpha4beta7 binding to MAdCAM-1, fibronectin, or VCAM-1. Recombinant human/mouse beta7 chimeras will be used to map epitopes recognized by function-perturbing anti-beta7 antibodies in order to identify regions likely to be involved in the binding of specific ligands. Since both alpha4beta7 and alpha4beta1 bind to fibronectin and VCAM-1, but only alpha4beta7 binds to MAdCAM-1, beta7/beta1 chimeras will be used to analyze regions of beta7 likely to be specifically required for MAdCAM-1 binding. Aim (iii) is to analyze the role of the other known beta7 integrin, alphaMLAbeta7, in lymphocyte interactions with epithelial cells and extracellular matrix proteins. alphaMLAbeta7 is selectively expressed by lymphocytes from the lung and other mucosal organs and is believed to mediate lymphocyte binding to gut epithelium. Proposed studies will investigate the role of alphaMLAbeta7 in binding of lymphocytes to epithelium from the airway and other mucosal and non-mucosal sites, and will attempt to identify alphaMLAbeta7 ligands.